Saturday, November 29, 2014

Lateral migration of a microdroplet under optical forces in a uniform flow

Hyunjun Cho, Cheong Bong Chang, Jin Ho Jung and Hyung Jin Sung

The behavior of a microdroplet in a uniform flow and subjected to a vertical optical force applied by a loosely focused Gaussian laser beam was studied numerically. The lattice Boltzmann method was applied to obtain the two-phase flow field, and the dynamic ray tracing method was adopted to calculate the optical force. The optical forces acting on the spherical droplets agreed well with the analytical values. The numerically predicted droplet migration distances agreed well with the experimentally obtained values. Simulations of the various flow and optical parameters showed that the droplet migration distance nondimensionalized by the droplet radius is proportional to the S number (zd /rp = 0.377S), which is the ratio of the optical force to the viscous drag. The effect of the surface tension was also examined. These results indicated that the surface tension influenced the droplet migration distance to a lesser degree than the flow and optical parameters. The results of the present work hold for the refractive indices of the mean fluid and the droplet being 1.33 and 1.59, respectively.


Dual Binding of an Antibody and a Small Molecule Increases the Stability of TERRA G-Quadruplex

Philip M. Yangyuoru, Dr. Marco Di Antonio, Chiran Ghimire, Giulia Biffi, Prof. Shankar Balasubramanian, and Prof. Hanbin Mao

In investigating the binding interactions between the human telomeric RNA (TERRA) G-quadruplex (GQ) and its ligands, it was found that the small molecule carboxypyridostatin (cPDS) and the GQ-selective antibody BG4 simultaneously bind the TERRA GQ. We previously showed that the overall binding affinity of BG4 for RNA GQs is not significantly affected in the presence of cPDS. However, single-molecule mechanical unfolding experiments revealed a population (48 %) with substantially increased mechanical and thermodynamic stability. Force-jump kinetic investigations suggested competitive binding of cPDS and BG4 to the TERRA GQ. Following this, the two bound ligands slowly rearrange, thereby leading to the minor population with increased stability. Given the relevance of G-quadruplexes in the regulation of biological processes, we anticipate that the unprecedented conformational rearrangement observed in the TERRA-GQ–ligand complex may inspire new strategies for the selective stabilization of G-quadruplexes in cells.


Elucidating Microscopic Structure and Dynamics in Optically Tweezed Environments

Debjit Roy, Dipankar Mondal, Debabrata Goswami

To probe the structure and dynamics of molecules under optical trapping conditions, we exploit the effect of femtosecond Fluorescence Resonance Energy Transfer (FRET) between dye molecules coated on the surface of polystyrene microspheres of various sizes suspended in water. The use of femtosecond laser pulses enables sensitive detection through two-photon fluorescence (TPF). Unlike conventional backscatter signal, the TPF signal shows a slow counterintuitive decay for the trapped microspheres when they are not fully within the laser illuminated volume. This decay is a characteristic sign of the occurrence of the FRET process. For microspheres with sizes less than the trapping focal volume, trapping of multiple particles can occur leading to the formation of optically bound clusters. Using different laser polarizations, we also extract information about the structure and dynamics of such optically bound clusters as a consequence of FRET.


Friday, November 28, 2014

Optical forces in nanoplasmonic systems: How do they work, what can they be useful for?

Olivier Martin, T.V. Raziman and R.I. Wolke

In this article, we share our vision for a future nanofactory, where plasmonic trapping is used to control the different manufacturing steps associated with the transformation of initial nanostructures to produce complex compounds. All the different functions existing in a traditional factory can be translated at the nanoscale using the optical forces produced by plasmonic nanostructures. A detailed knowledge of optical forces in plasmonic nanostructures is however essential to design such a nanofactory. To this end, we review the numerical techniques for computing optical forces on nanostructures immersed in a strong optical field and show under which conditions approximate solutions, like the dipole approximation, can be used in a satisfactory manner. Internal optical forces on realistic plasmonic antennas are investigated and the reconfiguration of a Fano-resonant plasmonic system using such internal forces is also studied in detail.


Wednesday, November 26, 2014

Optical rotation by plasmonic circular dichroism of isolated gold nanorod aggregates

Kamalesh Chaudhari and Thalappil Pradeep

We show that plasmonic chirality in single gold nanorod (GNR) aggregates leads to the rotation of polarization of the scattered light. 3D glasses in conjunction with linearly polarized dark field scattering microspectroscopy were used to study the chirality of single GNR aggregates. Using this hetero-polarizer setup, we not only detect but also quantify their chirality. A polar mapping strategy was used for providing direct evidence for the emergence of light of different polarization angles when chiral GNR aggregates were excited with circularly polarized light of different handedness. Further, we have developed a methodology to eliminate fluctuations in the scattering intensity by averaging and normalizing the data. This allows calculation of plasmonic circular dichroism scattering spectra with high accuracy.


Tight focusing of a radially polarized Laguerre–Bessel–Gaussian beam and its application to manipulation of two types of particles

Zhongquan Nie, Guang Shi, Dongyu Li, Xueru Zhang, Yuxiao Wang, Yinglin Song

The intensity distributions near the focus for radially polarized Laguerre–Bessel–Gaussian beams by a high numerical aperture objective in the immersion liquid are computed based on the vector diffraction theory. We compare the focusing properties of the radially polarized Laguerre–Bessel–Gaussian beams with those of Laguerre–Gaussian and Bessel–Gaussian modes. Furthermore, the effects of the optimally designed concentric three-zone phase filters on the intensity profiles in the focal region are examined. We further analyze the radiation forces on Rayleigh particles produced by the highly focused radially polarized Laguerre–Bessel–Gaussian beams using the specially engineered three-zone phase filters.


Dynamic operation of optical fibres beyond the single-mode regime facilitates the orientation of biological cells

Moritz Kreysing, Dino Ott, Michael J. Schmidberger, Oliver Otto, Mirjam Schürmann, Estela Martín-Badosa, Graeme Whyte & Jochen Guck

The classical purpose of optical fibres is delivery of either optical power, as for welding, or temporal information, as for telecommunication. Maximum performance in both cases is provided by the use of single-mode optical fibres. However, transmitting spatial information, which necessitates higher-order modes, is difficult because their dispersion relation leads to dephasing and a deterioration of the intensity distribution with propagation distance. Here we consciously exploit the fundamental cause of the beam deterioration—the dispersion relation of the underlying vectorial electromagnetic modes—by their selective excitation using adaptive optics. This allows us to produce output beams of high modal purity, which are well defined in three dimensions. The output beam distribution is even robust against significant bending of the fibre. The utility of this approach is exemplified by the controlled rotational manipulation of live cells in a dual-beam fibre-optical trap integrated into a modular lab-on-chip system.


Latest achievements in generalized Lorenz-Mie theories: A commented reference database

G. Gouesbet

Generalized Lorenz-Mie theories form a set of analytical approaches dealing with the interaction between electromagnetic arbitrary shaped beams and a class of particles possessing enough symmetries to allow one to use the method of separation of variables. This paper provides a commented reference database concerning generalized Lorenz-Mie theories for the period 2009-2013.


Monday, November 24, 2014

Manipulation of dielectric particles with nondiffracting parabolic beams

Antonio Ortiz-Ambriz, Julio C. Gutiérrez-Vega, and Dmitri Petrov

The trapping and manipulation of microscopic particles embedded in the structure of nondiffracting parabolic beams is reported. The particles acquire orbital angular momentum and exhibit an open trajectory following the parabolic fringes of the beam. We observe an asymmetry in the terminal velocity of the particles caused by the counteracting gradient and scattering forces.


Sunday, November 23, 2014

FDTD analysis of optical forces on bowtie antennas for high-precision trapping of nanostructures

Arif E. Cetin

We theoretically investigate the optical forces generated by a high near-field resolution antenna system through finite difference time domain calculations along with the Maxwell stress tensor method. Our antenna choice is bowtie-shaped nanostructures with small gap regions, exploiting propagating waveguide modes as well as localized surface plasmons. Our analysis shows that the antenna system supports large optical forces at the resonance wavelength where the near-field intensities as well as their gradients are the largest within the gap region. We show that the system exhibits much larger optical forces when the incident light polarization is along the bowtie gap as the system can effectively leverage the gap effect, compared to the case when the system is under the polarization normal to the gap. We also investigate the forces on a dielectric bead in the vicinity of the antennas for different positions to show the optical force characteristics of the bowtie-shaped antennas. Finally, the force analysis on different bead radiuses demonstrates the trapping efficiency of our antenna system.


Photophoretic trapping-Raman spectroscopy for single pollens and fungal spores trapped in air

Chuji Wang, Yong-Le Pan, Steven C. Hill, Brandon Redding

Photophoretic trapping-Raman spectroscopy (PTRS) is a new technique for measuring Raman spectra of particles that are held in air using photophoretic forces. It was initially demonstrated with Raman spectra of strongly-absorbing carbon nanoparticles (Pan et al. [44] (Opt Express 2012)). In the present paper we report the first demonstration of the use of PTRS to measure Raman spectra of absorbing and weakly-absorbing bioaerosol particles (pollens and spores). Raman spectra of three pollens and one smut spore in a size range of 6.2–41.8 µm illuminated at 488 nm are shown. Quality spectra were obtained in the Raman shift range of 1600–3400 cm−1 in this exploratory study. Distinguishable Raman scattering signals with one or a few clear Raman peaks for all four aerosol particles were observed within the wavenumber region 2940–3030 cm−1. Peaks in this region are consistent with previous reports of Raman peaks in the 1600–3400 cm−1 range for pollens and spores excited at 514 nm measured by a conventional Raman spectrometer. Noise in the spectra, the fluorescence background, and the weak Raman signals in most of the 1600–3400 cm−1 region make some of the spectral features barely discernable or not discernable for these bioaerosols except the strong signal within 2940–3030 cm−1. Up to five bands are identified in the three pollens and only two bands appear in the fungal spore, but this may be because the fungal spore is so much smaller than any of the pollens. The fungal spore signal relative to the air-nitrogen Raman band is approximately 10 times smaller than that ratio for the pollens. The five bands are tentatively assigned to the CH2 symmetric stretch at 2948 cm−1, CH2 Fermi resonance stretch at 2970 cm−1, CH3 symmetric stretch at 2990 cm−1, CH3 out-of-plane end asymmetric stretch at 3010 cm−1, and unsaturated =CH stretch at 3028 cm−1. The two dominant bands of the up-to-five Raman bands in the 2940–3030 cm−1 region have a consistent band spacing of 25 cm−1 in all four aerosols. Finally we discuss improvements to the PTRS that should provide a system which can trap a higher fraction of particle types and obtain Raman spectra over a larger range (e.g., 200–3600 cm−1) than those achieved here.


Collective flow dynamics across a bacterial carpet: Understanding the forces generated

Yi-Teng Hsiao, Jing-Hui Wang, Kuan-Ting Wu, Jengjan Tsai, Cheng-Hung Chang and Wei-Yen Woon

Bacterial carpets consist of randomly anchored uni-polar-flagellated sodium-motive bacterial matrix are prepared by flow deposition. Collective flow dynamics across the bacterial carpets are probed with optical tweezers-microsphere assay. Around the center of a uniform bacterial cluster, collective forces that pull microsphere towards carpet surface are detected at a distance of 10 μm away from carpets. At sodium-motive driving over a critical value, the force magnitudes increase abruptly, suggesting a threshold-like transition of hydrodynamic synchronization across bacterial carpet. The abrupt force increase is explained in term of bifurcation to phase synchronization in a noisy non-linearly coupled rotor array mediated by hydrodynamic interactions.


Characterizing conical refraction optical tweezers

C. McDonald, C. McDougall, E. Rafailov, and D. McGloin

Conical refraction occurs when a beam of light travels through an appropriately cut biaxial crystal. By focusing the conically refracted beam through a high numerical aperture microscope objective, conical refraction optical tweezers can be created, allowing for particle manipulation in both Raman spots, and in the Lloyd/Poggendorff rings. We present a thorough quantification of the trapping properties of such a beam, focusing on the trap stiffness, and how this varies with trap power and trapped particle location. We show that the lower Raman spot can be thought of as a single-beam optical gradient force trap, while radiation pressure dominates in the upper Raman spot, leading to optical levitation rather than trapping. Particles in the Lloyd/Poggendorff rings experience a lower trap stiffness than particles in the lower Raman spot, but benefit from rotational control.


Friday, November 21, 2014

Template stripped double nanohole in a gold film for nano-optical tweezers

Ana Zehtabi-Oskuie, Aurora A Zinck, Ryan M Gelfand and Reuven Gordon

Double nanohole (DNH) laser tweezers can optically trap and manipulate objects such as
proteins, nanospheres, and other nanoparticles; however, precise fabrication of those DNHs has been expensive with low throughput. In this work, template stripping was used to pattern DNHs with gaps as small as 7 nm, in optically thick Au films. These DNHs were used to trap streptavidin as proof of operation. The structures were processed multiple times from the same template to demonstrate reusability. Template stripping is a promising method for high-throughput, reproducible, and cost efficient fabrication of DNH apertures for optical trapping.


Cleaved fiber optic double nanohole optical tweezers for trapping nanoparticles

Ryan M. Gelfand, Skylar Wheaton, and Reuven Gordon
We demonstrate the trapping of single 20 and 40 nm polystyrene spheres at the cleaved end of a fiber optic with a double nanohole aperture in gold and without any microscope optics. An optical transmission increase of 15% indicates a trapping event for the 40 nm particle, and the jump is 2% for the 20 nm particle. This modular technique can be used to replace those used with current optical trapping setups that require complicated free space optics and frequent calibration, with one that is modular and requires no free space optics. This simple arrangement with the potential for fiber translation is of interest for future biosensor and optical nano-pipette devices.


Optical sorting of nonspherical and living microobjects in moving interference structures

Petr Jákl, Alejandro V. Arzola, Martin Šiler, Lukáš Chvátal, Karen Volke-Sepúlveda, and Pavel Zemánek

Contactless, sterile and nondestructive separation of microobjects or living cells is demanded in many areas of biology and analytical chemistry, as well as in physics or engineering. Here we demonstrate advanced sorting methods based on the optical forces exerted by travelling interference fringes with tunable periodicity controlled by a spatial light modulator. Besides the sorting of spherical particles we also demonstrate separation of algal cells of different sizes and particles of different shapes. The three presented methods offer simultaneous sorting of more objects in static suspension placed in a Petri dish or on a microscope slide.


Thursday, November 20, 2014

An integrated centrifugo-opto-microfluidic platform for arraying, analysis, identification and manipulation of individual cells

R. Burger, D. Kurzbuch, R. Gorkin, G. Kijanka, M. Glynn, C. McDonagh and J. Ducrée

In this work we present a centrifugal microfluidic system enabling highly efficient collective trapping and alignment of particles such as microbeads and cells, their multi-colour fluorescent detection and subsequent manipulation by optical tweezers. We demonstrate array-based capture and imaging followed by “cherry-picking” of individual particles, first for fluorescently labelled polystyrene (PS) beads and then for cells. Different cell lines are discriminated based on intracellular as well as surface-based markers.


Birefringence of a normal human red blood cell and related optomechanics in an optical trap

Belavadi Venkatakrishnaiah Nagesh; Yogesha; Ramarao Pratibha; Praveen Parthasarathi; Shruthi Subhash Iyengar; Sarbari Bhattacharya; Sharath Ananthamurthy

A normal human red blood cell (RBC) when trapped with a linearly polarized laser, reorients about the electric polarization direction and then remains rotationally bound to this direction. This behavior is expected for a birefringent object. We have measured the birefringence of distortion-free RBCs in an isotonic medium using a polarizing microscope. The birefringence is confined to the cell’s dimple region and the slow axis is along a diameter. We report an average retardation of 3.5±1.5  nm for linearly polarized green light (λ=546  nm). We also estimate a retardation of 1.87±0.09  nm from the optomechanical response of the RBC in an optical trap. We reason that the birefringence is a property of the cell membrane and propose a simple model attributing the origin of birefringence to the phospholipid molecules in the lipid bilayer and the variation to the membrane curvature. We observe that RBCs reconstituted in shape subsequent to crenation show diminished birefringence along with a sluggish optomechanical response in a trap. As the arrangement of phospholipid molecules in the cell membrane is disrupted on crenation, this lends credence to our conjecture on the origin of birefringence. Dependence of the birefringence on membrane contours is further illustrated through studies on chicken RBCs.


Structural Rearrangements in CHO Cells After Disruption of Individual Cytoskeletal Elements and Plasma Membrane

Špela Zemljič Jokhadar, Jure Derganc

Cellular structural integrity is provided primarily by the cytoskeleton, which comprises microtubules, actin filaments, and intermediate filaments. The plasma membrane has been also recognized as a mediator of physical forces, yet its contribution to the structural integrity of the cell as a whole is less clear. In order to investigate the relationship between the plasma membrane and the cytoskeleton, we selectively disrupted the plasma membrane and each of the cytoskeletal elements in Chinese hamster ovary cells and assessed subsequent changes in cellular structural integrity. Confocal microscopy was used to visualize cytoskeletal rearrangements, and optical tweezers were utilized to quantify membrane tether extraction. We found that cholesterol depletion from the plasma membrane resulted in rearrangements of all cytoskeletal elements. Conversely, the state of the plasma membrane, as assessed by tether extraction, was affected by disruption of any of the cytoskeletal elements, including microtubules and intermediate filaments, which are located mainly in the cell interior. The results demonstrate that, besides the cytoskeleton, the plasma membrane is an important contributor to cellular integrity, possibly by acting as an essential framework for cytoskeletal anchoring. In agreement with the tensegrity model of cell mechanics, our results support the notion of the cell as a prestressed structure.


Whirl-enhanced continuous wave laser trapping of particles

Stanislaw Bartkiewicz and Andrzej Miniewicz

Tightly focused laser beams can trap micro- and nanoparticles suspended in liquids in their focal spots enabling different functionalities including 3D manipulations and assembling. Here, we report on a remarkable strength liquid-liquid phase separation and crystallization experiments in para-nitroaniline dissolved in 1,4-dioxane. For optical trapping of the para-nitroaniline we used partially absorbed by solute low-power, weakly focused light beam from continuous-wave laser. The experiments were performed for solution deposited on glass with upper free-surface and solution contained between two glass plates. The usual gradient field force and scattering force solely are insufficient to properly describe the observed particle gathering effects extending far beyond the optical trap potential. The concept of whirl-enhanced and temperature assisted optical trapping is postulated. The relative simplicity of the used geometry for trapping will broaden the understanding of the light-matter interaction and promises widespread application of the observed effect in optically controlled crystallization.


Tuesday, November 18, 2014

Energy flow between two hydrodynamically coupled particles kept at different effective temperatures

A. Bérut, A. Petrosyan and S. Ciliberto

We measure the energy exchanged between two hydrodynamically coupled micron-sized Brownian particles trapped in water by two optical tweezers. The system is driven out of equilibrium by random-forcing the position of one of the two particles. The forced particle behaves as it has an "effective temperature" higher than that of the other bead. This driving modifies the equilibrium variances and cross-correlation functions of the bead positions: we measure an energy flow between the particles and an instantaneous cross-correlation, proportional to the effective temperature difference between the two particles. A model of the interaction which is based on classical hydrodynamic coupling tensors is proposed. The theoretical and experimental results are in excellent agreement.


Low-temperature Bessel beam trap for single submicrometer aerosol particle studies

Jessica W. Lu, Merrill Isenor, Egor Chasovskikh, David Stapfer and Ruth Signorell

We report on a new instrument for single aerosol particle studies at low temperatures that combines an optical trap consisting of two counter-propagating Bessel beams (CPBBs) and temperature control down to 223 K (−50 °C). The apparatus is capable of capturing and stably trapping individual submicrometer- to micrometer-sized aerosol particles for up to several hours. First results from studies of hexadecane, dodecane, and water aerosols reveal that we can trap and freeze supercooled droplets ranging in size from ∼450 nm to 5500 nm (radius). We have conducted homogeneous and heterogeneous freezing experiments, freezing-melting cycles, and evaporation studies. To our knowledge, this is the first reported observation of the freezing process for levitated single submicrometer-sized droplets in air using optical trapping techniques. These results show that a temperature-controlled CPBB trap is an attractive new method for studying phase transitions of individual submicrometer aerosol particles.


Realization of nonequilibrium thermodynamic processes using external colored noise

Pau Mestres, Ignacio A. Martinez, Antonio Ortiz-Ambriz, Raul A. Rica, and Edgar Roldan

We investigate the dynamics of single microparticles immersed in water that are driven out of equilibrium in the presence of an additional external colored noise. As a case study, we trap a single polystyrene particle in water with optical tweezers and apply an external electric field with flat spectrum but a finite bandwidth of the order of kHz. The intensity of the external noise controls the amplitude of the fluctuations of the position of the particle and therefore of its effective temperature. Here we show, in two different nonequilibrium experiments, that the fluctuations of the work done on the particle obey the Crooks fluctuation theorem at the equilibrium effective temperature, given that the sampling frequency and the noise cutoff frequency are properly chosen.


A method for an approximate determination of a polymer-rich-domain concentration in phase-separated poly(N-isopropylacrylamide) aqueous solution by means of confocal Raman microspectroscopy combined with optical tweezers

Tatsuya Shoji, Riku Nohara, Noboru Kitamura, Yasuyuki Tsuboi
The paper demonstrates that a confocal Raman microspectroscope combined with optical tweezers is a promising technique to estimate polymer concentration in polymer-rich domain in phase-separated-aqueous polymer solution. The sample polymer is poly-(N-isopropylacrylamide) (PNIPAM) that is well-known as a representative thermo-responsive polymer. Optical tweezers can selectively trap the polymer-rich domain at the focal point in non-contact and non-intrusive modes. Such situation allows us to determine polymer concentration in the domain, which has been unclear due to a lack of appropriate analytical technique. It is applicable for a variety of other thermo-responsive polymers.


Pulling extremely anisotropic lossy particles using light without intensity gradient

Andrey Novitsky and Cheng-Wei Qiu

We study the effect of pulling optical force acting on a nonmagnetic anisotropic bead in electromagnetic fields without intensity gradient. Extreme anisotropy can be realized by a hyperbolic metamaterial made of metal-dielectric multilayers. We find that a passive anisotropic Rayleigh particle cannot be pulled by the electromagnetic beam without intensity gradient and the nonparaxial incident beams can exert backward negative force acting on anisotropic dipole spheres. We investigate the validity of the dipole approximation and establish the conditions for pulling hyperbolic-metamaterial particles. It is important to note that the loss in hyperbolic metamaterial does not suppress the effect of pulling force. We notice that the nonradial components of the permittivity tensor strongly affect the optical force and propose the way of material engineering to ensure the optical pulling.


Nano-optomechanics with optically levitated nanoparticles

Levi P. Neukirch & A. Nick Vamivakas

Nano-optomechanics is a vibrant area of research that continues to push the boundary of quantum science and measurement technology. Recently, it has been realised that the optical forces experienced by polarisable nanoparticles can provide a novel platform for nano-optomechanics with untethered mechanical oscillators. Remarkably, these oscillators are expected to exhibit quality factors approaching . The pronounced quality factors are a direct result of the mechanical oscillator being freed from a supporting substrate. This review provides an overview of the basic optical physics underpinning optical trapping and optical levitation experiments, it discusses a number of experimental approaches to optical trapping and finally outlines possible applications of this nano-optomechanics modality in hybrid quantum systems and nanoscale optical metrology.


Monday, November 17, 2014

Optical trapping of core-shell magnetic microparticles by cylindrical vector beams

Min-Cheng Zhong, Lei Gong, Di Li, Jin-Hua Zhou, Zi-Qiang Wang and Yin-Mei Li

Optical trapping of core-shell magnetic microparticles is experimentally demonstrated by using cylindrical vector beams. Second, we investigate the optical trapping efficiencies. The results show that radially and azimuthally polarized beams exhibit higher axial trapping efficiencies than the Gaussian beam. Finally, a trapped particle is manipulated to kill a cancer cell. The results make possible utilizing magnetic particles for optical manipulation, which is an important advantage for magnetic particles as labeling agent in targeted medicine and biological analysis.


Upconversion particle as a local luminescent Brownian probe - a photonic force microscopy study

Flavio M. Mor, Andrzej Sienkiewicz, László Forró, and Sylvia Jeney

Near-infrared (NIR) light sensitive lanthanide-doped NaYF4 upconversion particles (UCPs) are gaining increasing attention as local probes in biomedical applications. Here, we implemented a photonic force microscope (PFM) to manipulate and study the optical properties of trapped single UCPs, β-NaYF4:Yb,Er. In particular, we focused on the mechanisms of the optical trapping of spherical and non-spherical UCPs of different sizes, in the range of 0.5 - 2 μm, as well as on their upconversion photoluminescence (UCL) properties under excitation with a strongly focused laser beam (λ = 1064 nm) of the PFM, operating at power densities up to 14.7 MW cm−2. A careful analysis of UCL under such conditions points to three emission peaks at 469, 503.6 and 616.1 nm, which were not reported before. The analysis of Brownian motion was used to quantify the thermal fluctuations of the particle inside the optical trap as well as the particle sizes and optical forces acting in two dimensions perpendicular to the optical axis. A steep dependence of UCL as a function of the particle diameter was found for UCPs having sizes smaller than the focal spot (900 nm) of the NIR laser.


Unveiling the correlation between non-diffracting tractor beam and its singularity in Poynting vector

Dongliang Gao, Andrey Novitsky, Tianhang Zhang, Fook Chiong Cheong, Lei Gao, Chwee Teck Lim, Boris Luk'yanchuk and Cheng-Wei Qiu

This paper investigates the singular optics of nonparaxial light beams in the near field when the light behaves as a tractor beam. New insights into the optical pulling force, which is usually represented by integrating the stress tensor at a black box enclosing the object, are interpreted by the optical singularity of the Poynting vector. The negative nonconservative pulling force originates from the transfer of the azimuthal Poynting vector to the longitudinal component partly owing to the presence of a scatterer. The separatrice pattern and singularity shifts of the Poynting vector unanimously exhibit a differentiable near-field distribution in the presence of optical pulling force. A new method is established to calculate the near-field optical force using the differential Poynting vector in the far field. The results obtained provide a clear physical interpretation of the light–matter interaction and manifest the significance of singular optics in manipulating objects.


Three-dimensional microfabrication using local electrophoresis deposition and a laser trapping technique

Takanari Takai, Hidenobu Nakao, and Futoshi Iwata

We describe a novel fabrication method of three-dimensional (3D) microstructures using local electrophoresis deposition together with laser trapping. A liquid cell consisting of two-faced conductive substrates was filled with a colloidal solution of Au nanoparticles. The nanoparticles were trapped by a laser spot and positioned on the bottom substrate, then deposited onto the surface by the application of electrical voltage between the two substrates. By moving the liquid cell downward while maintaining the deposition, 3D microstructures were successfully fabricated. The smallest diameter of the fabricated pillar was 500 nm, almost the same as that of the Airy disc. The Young’s modulus of the fabricated structure was 1.5 GPa.


Sunday, November 16, 2014

Comparative study of methods to calibrate the stiffness of a single-beam gradient-force optical tweezers over various laser trapping powers

Mohammad Sarshar; Winson T. Wong; Bahman Anvari

Optical tweezers have become an important instrument in force measurements associated with various physical, biological, and biophysical phenomena. Quantitative use of optical tweezers relies on accurate calibration of the stiffness of the optical trap. Using the same optical tweezers platform operating at 1064 nm and beads with two different diameters, we present a comparative study of viscous drag force, equipartition theorem, Boltzmann statistics, and power spectral density (PSD) as methods in calibrating the stiffness of a single beam gradient force optical trap at trapping laser powers in the range of 0.05 to 1.38 W at the focal plane. The equipartition theorem and Boltzmann statistic methods demonstrate a linear stiffness with trapping laser powers up to 355 mW, when used in conjunction with video position sensing means. The PSD of a trapped particle’s Brownian motion or measurements of the particle displacement against known viscous drag forces can be reliably used for stiffness calibration of an optical trap over a greater range of trapping laser powers. Viscous drag stiffness calibration method produces results relevant to applications where trapped particle undergoes large displacements, and at a given position sensing resolution, can be used for stiffness calibration at higher trapping laser powers than the PSD method.


Direct 2D measurement of time-averaged forces and pressure amplitudes in acoustophoretic devices using optical trapping

Stefan Lakämper, Andreas Lamprecht, Iwan A. T. Schaap and Jurg Dual

Ultrasonic standing waves are increasingly applied in the manipulation and sorting of micrometer-sized particles in microfluidic cells. To optimize the performance of such devices, it is essential to know the exact forces that the particles experience in the acoustic wave. Although much progress has been made via analytical and numerical modeling, the reliability of these methods relies strongly on the assumptions used, e.g. the boundary conditions. Here, we have combined an acoustic flow cell with an optical laser trap to directly measure the force on a single spherical particle in two dimensions. While performing ultrasonic frequency scans, we measured the time-averaged forces on single particles that were moved with the laser trap through the microfluidic cell. The cell including piezoelectric transducers was modeled with finite element methods. We found that the experimentally obtained forces and the derived pressure fields confirm the predictions from theory and modeling. This novel approach can now be readily expanded to other particle, chamber, and fluid regimes and opens up the possibility of studying the effects of the presence of boundaries, acoustic streaming, and non-linear fluids.


Analysis of radiation pressure induced nonlinear optofluidics

Yong Xu, Peng Zhang, Sunghwan Jung, and Aram Lee

We analyze two nonlinear optofluidic processes where nonlinearity is induced by the interplay between optical field and liquid interface. Specifically, guided optical waves generate radiation pressure on the liquid interface, which can in turn distort the liquid interface and modify the properties of the optical field. In the first example, we discuss the feasibility of nonlinear optofluidic solitons, where optical field is governed by the nonlinear Schrödinger equation and nonlinearity is effectively determined by liquid properties. Then, we analyze a nonlinear optofluidic process associated with a high quality (Q) factor whispering gallery mode (WGM) in a liquid droplet. Similar to Kerr effects, the WGM can produce a frequency shift proportional to the WGM power. Using liquid properties that are experimentally attainable, we find that it may only take a few photons to generate measurable WGM resonance shift. Such a possibility may eventually lead to nonlinear optics at single photon energy level.


Saturday, November 8, 2014

Dynamic particle tracking via temporal focusing multiphoton microscopy with astigmatism imaging

Chi-Hsiang Lien, Chun-Yu Lin, Shean-Jen Chen, and Fan-Ching Chien

A three-dimensional (3D) single fluorescent particle tracking strategy based on temporal focusing multiphoton excitation microscopy (TFMPEM) combined with astigmatism imaging is proposed for delivering nanoscale-level axial information that reveals 3D trajectories of single fluorospheres in the axially-resolved multiphoton excitation volume without z-axis scanning. Whereas other scanning spatial focusing multiphoton excitation schemes induce optical trapping interference, temporal focusing multiphoton excitation produces widefield illumination with minimum optical trapping force on the fluorospheres. Currently, the lateral and axial positioning resolutions of the dynamic particle tracking approach are about 14 nm and 21 nm in standard deviation, respectively. Furthermore, the motion behavior and diffusion coefficients of fluorospheres in glycerol solutions with different concentrations are dynamically measured at a frame rate up to 100 Hz. This TFMPEM with astigmatism imaging holds great promise for exploring dynamic molecular behavior deep inside biotissues via its superior penetration, reduced trapping effect, fast frame rate, and nanoscale-level positioning.


Rotation dynamics of a uniaxial birefringent cylinder in an optical tweezer with a rotating polarization ellipse

Huachao Yu and Weilong She

We make a theoretical investigation on the rotation dynamics of a uniaxial birefringent cylinder in an optical tweezer with a rotating polarization ellipse. Analytical results have been obtained for the cylinder suspended in fluid with a low Reynolds number. They show that there are two distinct rotational states of the cylinder. In one state, the optical axis of the cylinder rotates synchronously with the rotating polarization ellipse. In another state, the optical axis rotates at a time-periodic angular velocity. We find that such a rotation feature is rooted in the angular dependence of the radiation torque exerted on the cylinder. We also notice that the transmitted light through the cylinder carries the information of rotation angle of the cylinder, which enables one to measure quantities such as the cylinder’s angular velocity and its period by detecting the power of the transmitted light.


Universal, strong and long-ranged trapping by optical conveyors

David B. Ruffner and David G. Grier

Optical conveyors are active tractor beams that selectively transport illuminated objects either upstream or downstream along their axes. Formed by the coherent superposition of coaxial Bessel beams, an optical conveyor features an axial array of equally spaced intensity maxima that act as optical traps for small objects. We demonstrate through measurements on colloidal spheres and numerical calculations based on the generalized Lorenz-Mie theory that optical conveyors’ interferometric structure endows them with trapping characteristics far superior to those of conventional optical tweezers. Optical conveyors form substantially stiffer traps and can transport a wider variety of materials over a much longer axial range.


Friday, November 7, 2014

Optofluidic taming of a colloidal dimer with a silicon nanocavity

C. Pin, B. Cluzel, C. Renaut, D. Peyrade, E. Picard, E. Hadji and F. de Fornel

We report here the optical trapping of a heterogeneous colloidal dimer above a photonic crystal nanocavity used as an on-chip optical tweezer. The trapped dimer consists of a cluster of two dielectric microbeads of different sizes linked by van der Waals forces. The smallest bead, 1 μm in diameter, is observed to be preferentially trapped by the nanotweezer, leaving the second bead untrapped. The rotational nature of the trapped dimer Brownian motion is first evidenced. Then, in the presence of a fluid flow, control of its orientation and rotation is achieved. The whole system is found to show high rotational degrees of freedom, thereby acting as an effective flow-sensitive microscopic optical ball joint.


Structure and function of Enterotoxigenic Escherichia coli fimbriae from differing assembly pathways

Narges Mortezaei, Chelsea R. Epler, Paul P. Shao, Mariam Shirdel, Bhupender Singh, Annette McVeigh, Bernt Eric Uhlin, Stephen J. Savarino, Magnus Andersson and Esther Bullitt

Pathogenic enterotoxigenic Escherichia coli (ETEC) are the major bacterial cause of diarrhea in young children in developing countries and in travelers, causing significant mortality in children. Adhesive fimbriae are a prime virulence factor for ETEC, initiating colonization of the small intestinal epithelium. Similar to other Gram-negative bacteria, ETEC express one or more diverse fimbriae, some assembled by the chaperone-usher pathway and others by the alternate chaperone pathway. Here we elucidate structural and biophysical aspects and adaptations of each fimbrial type to its respective host niche. CS20 fimbriae are compared to CFA/I fimbriae, which are two ETEC fimbriae assembled via different pathways, and to P-fimbriae from uropathogenic E. coli. Many fimbriae unwind from their native helical filament to an extended linear conformation under force, thereby sustaining adhesion by reducing load at the point of contact between the bacterium and the target cell. CFA/I fimbriae require the least force to unwind, followed by CS20 fimbriae and then P-fimbriae, which require the highest unwinding force. We conclude from our electron microscopy reconstructions, modeling, and force spectroscopy data that the target niche plays a central role in the biophysical properties of fimbriae that are critical for bacterial pathophysiology.


The use of optical trap and microbeam to investigate the mechanical and transport characteristics of tunneling nanotubes in tumor spheroids

Pooja Patheja, Raktim Dasgupta, Alok Dube, Sunita Ahlawat, Ravi Shanker Verma and Pradeep Kumar Gupta

Manipulation of cells at the surface of tumor spheroid using optical tweezers and injection of fluorescent dye into a trapped cell using optoporation technique.
The use of optical trap and microbeam for investigating mechanical and transport properties of inter cellular tunneling nanotubes (TnTs) in tumor spheroids has been demonstrated. TnTs in tumor spheroids have been visualized by manipulating TnT connected cells using optical tweezers. Functionality of the TnTs for transferring cytoplasmic vesicles and injected dye molecules by optoporation method has been studied. Further, the TnTs could be longitudinally stretched by manipulating the connected cells and their elastic response was studied.


Thursday, November 6, 2014

Levitation of finite-size electric dipole over epsilon-near-zero metamaterial

Sergey Krasikov, Ivan V. Iorsh, Alexander Shalin and Pavel A. Belov

Following the suggestion of Rodriguez-Fortuno et al. [Phys. Rev. Lett. 112, 033902 (2014)], we study the repulsive force acting on a electric dipole placed over a surface of epsilon-near-zero (ENZ) metamaterial. The dependence of the repulsive force value on the dipole size has been studied. We show that the effect of finite size drastically affects the values of the repulsive force as compared to the point-dipole case.


Determination of motility forces on isolated chromosomes with laser tweezers

Nima Khatibzadeh, Alexander B. Stilgoe, Ann A. M. Bui, Yesenia Rocha, Gladys M. Cruz, Vince Loke, Linda Z. Shi, Timo A. Nieminen, Halina Rubinsztein-Dunlop & Michael W. Berns

Quantitative determination of the motility forces of chromosomes during cell division is fundamental to understanding a process that is universal among eukaryotic organisms. Using an optical tweezers system, isolated mammalian chromosomes were held in a 1064 nm laser trap. The minimum force required to move a single chromosome was determined to be ≈0.8–5 pN. The maximum transverse trapping efficiency of the isolated chromosomes was calculated as ≈0.01–0.02. These results confirm theoretical force calculations of ≈0.1–12 pN to move a chromosome on the mitotic or meiotic spindle. The verification of these results was carried out by calibration of the optical tweezers when trapping microspheres with a diameter of 4.5–15 µm in media with 1–7 cP viscosity. The results of the chromosome and microsphere trapping experiments agree with optical models developed to simulate trapping of cylindrical and spherical specimens.


The minimal cadherin-catenin complex binds to actin filaments under force

Craig D. Buckley, Jiongyi Tan, Karen L. Anderson, Dorit Hanein, Niels Volkmann, William I. Weis, W. James Nelson, Alexander R. Dunn

Cadherins are an ancient class of transmembrane proteins that are essential for the formation of multicellular tissues in metazoans. Cadherins link intercellular adhesions to the cellular cytoskeleton, but how they are connected specifically to actin filaments is a hotly debated issue. Genetic and cell culture experiments indicate that E-cadherin, β-catenin, and the actin filament binding protein αE-catenin form a minimal cadherin-catenin complex that binds to the actin cytoskeleton directly in epithelial tissues. However, experiments with purified proteins showed that a stable cadherin-catenin complex can be reconstituted, but it does not bind strongly to actin filaments in solution. Nevertheless, cell culture experiments indicated that the cadherin-catenin complex is under constitutive actomyosin-generated tension and that this connection is required for mechanotransduction at cadherin-based adhesions. Here, we tested the hypothesis that tension is required to stabilize a linkage between the cadherin-catenin complex and actin filaments, and clarify how the cadherin-catenin complex could interact directly with the actin cytoskeleton in cells.


Optical trapping in the presence of higher order mode sources and interactions

I Liberal, I Ederra, R Gonzalo and R W Ziolkowski

The force fields produced by higher order mode (HOM) TMen1 sources are investigated analytically and numerically. Their application to optical manipulation and trapping is emphasized. It is found that the strong reactive fields excited by these advanced HOM nanoantennas significantly enhance both the gradient and near-field absorption-scattering forces, as well as increase the range of attraction of those absorption-scattering forces. Moreover, the impact on these force fields by the excitation of HOMs in a nanoparticle is addressed. These results are used to demonstrate the theoretical possibility of the generation of tractor beams by highly subwavelength sources on resonant dipolar nanoparticles. The analysis reveals that the associated attractive forces can be enhanced by several orders of magnitude before the HOMs induce any noticeable degradation of the radiation efficiency. The practical implementation of HOM TMen1 sources by means of active core-shell resonators, as well as their potential for the self-induced trapping of fluorescent molecules, is also addressed.


Tuesday, November 4, 2014

All-fiber self-accelerating Bessel-like beam generator and its application

Zhihai Liu, Yaxun Zhang, Yu Zhang, Peibo Liang, Jun Yang, and Libo Yuan

We demonstrate an all-fiber transverse self-accelerating Bessel-like beam generator and its optical trapping application. The theoretical and experimental studies have been provided to verify this beam properties. We produce the Bessel-like beam by splicing the single-mode fiber and multimode fiber with a defined offset and then modulating the output light beam phase by fabricating a small hemispherical-lens fiber tip; therefore, the phase-modulated Bessel-like beam generates the properties of transverse self-accelerating. The transverse acceleration of the the Bessel-like beam generated here is ∼10−4  μm−1, which is almost 100 times larger than that of the beam generated in the free-space optical circuit based on the lens. The experimental and simulated results have good consistencies. The realization of the microparticle transverse acceleration transporting with this Bessel-like beam provides a new method for microparticles to be transported in a bending trajectory. This all-fiber transverse self-accelerating Bessel-like beam generator structure is simple, with high integration and small size, and constitutes a new development for high-precision biological cell experiments and manipulations.


Optical trapping reveals propulsion forces, power generation and motility efficiency of the unicellular parasites Trypanosoma brucei brucei

Eric Stellamanns, Sravanti Uppaluri, Axel Hochstetter, Niko Heddergott, Markus Engstler & Thomas Pfohl
Unicellular parasites have developed sophisticated swimming mechanisms to survive in a wide range of environments. Cell motility of African trypanosomes, parasites responsible for fatal illness in humans and animals, is crucial both in the insect vector and the mammalian host. Using millisecond-scale imaging in a microfluidics platform along with a custom made optical trap, we are able to confine single cells to study trypanosome motility. From the trapping characteristics of the cells, we determine the propulsion force generated by cells with a single flagellum as well as of dividing trypanosomes with two fully developed flagella. Estimates of the dissipative energy and the power generation of single cells obtained from the motility patterns of the trypanosomes within the optical trap indicate that specific motility characteristics, in addition to locomotion, may be required for antibody clearance. Introducing a steerable second optical trap we could further measure the force, which is generated at the flagellar tip. Differences in the cellular structure of the trypanosomes are correlated with the trapping and motility characteristics and in consequence with their propulsion force, dissipative energy and power generation.


Anomalous optical forces on the anisotropic Rayleigh particles

Y. X. Ni, J. K. Chen, and L. Gao
We investigate the optical forces on the radially anisotropic spheres from an incident plane wave based on our generalized full-wave scattering theory and the Maxwell stress tensor integration techniques. We demonstrate that the optical force on the Rayleigh sphere with radial anisotropy does not always obey the well-known Rayleigh’s law F~k40a6 (where k0 is the wave number and a is the radius of the sphere), but could be anomalous with the laws such as F~k00a2, F~k−20a0, and F~k80a10 under certain conditions. Therefore, the optical force on the anisotropic Rayleigh spheres is enhanced at the electric dipole resonance, and may be further increased by tuning the anisotropic parameters. On the contrary, the optical forces on the anisotropic spheres can be largely reduced for anisotropic spheres with electromagnetic transparency.


Monday, November 3, 2014

Squeezing red blood cells on an optical waveguide to monitor cell deformability during blood storage

Balpreet Singh Ahluwalia, Peter McCourt, Ana Oteiza, James S. Wilkinson, Thomas R Huser and Olav Gauta Helleso 

Red blood cells squeeze through micro-capillaries as part of blood circulation in the body. The deformability of red blood cells is thus critical for blood circulation. In this work, we report a method to optically squeeze red blood cells using the evanescent field present on top of a planar waveguide chip. The optical forces from a narrow waveguide are used to squeeze red blood cells to a size comparable to the waveguide width. Optical forces and pressure distributions on the cells are numerically computed to explain the squeezing process. The proposed technique is used to quantify the loss of blood deformability that occurs during blood storage lesion. Squeezing red blood cells using waveguides is a sensitive technique and works simultaneously on several cells, making the method suitable for monitoring stored blood.


Separation of blood cells with differing deformability using deterministic lateral displacement

David Holmes, Graeme Whyte, Joe Bailey, Nuria Vergara-Irigaray, Andrew Ekpenyong, Jochen Guck and Tom Duke

Determining cell mechanical properties is increasingly recognized as a marker-free way to characterize and separate biological cells. This emerging realization has led to the development of a plethora of appropriate measurement techniques. Here, we use a fairly novel approach, deterministic lateral displacement (DLD), to separate blood cells based on their mechanical phenotype with high throughput. Human red blood cells were treated chemically to alter their membrane deformability and the effect of this alteration on the hydrodynamic behaviour of the cells in a DLD device was investigated. Cells of defined stiffness (glutaraldehyde cross-linked erythrocytes) were used to test the performance of the DLD device across a range of cell stiffness and applied shear rates. Optical stretching was used as an independent method for quantifying the variation in stiffness of the cells. Lateral displacement of cells flowing within the device, and their subsequent exit position from the device were shown to correlate with cell stiffness. Data showing how the isolation of leucocytes from whole blood varies with applied shear rate are also presented. The ability to sort leucocyte sub-populations (T-lymphocytes and neutrophils), based on a combination of cell size and deformability, demonstrates the potential for using DLD devices to perform continuous fractionation and/or enrichment of leucocyte sub-populations from whole blood.


Microscale mapping of oscillatory flows

Spas Nedev, S. Carretero-Palacios, S. R. Kirchner, F. Jäckel and J. Feldmann

We present an optofluidic method that allows the two-dimensional vectorial near-field mapping of oscillatory flows with micron-scale resolution. An oscillatory flow created by a microsource (an optically trapped silica particle set to oscillate in a dipole-type mode) is detected by another twin silica particle independently trapped and located in the vicinity of the source. Fourier analysis of the motion of the detecting particle at different points in space and time renders the vectorial velocity map around the oscillating microsphere. The method introduced here paves the way for in-situ characterization of fast mixing microscale devices and for new detection methods able to provide location and recognition (due to the field pattern) of moving sources that may be applied to both artificial and living microobjects, including macromolecules, cells, and microorganisms.


Tailoring azimuthal optical force on lossy chiral particles in Bessel beams

Huajin Chen, Neng Wang, Wanli Lu, Shiyang Liu, and Zhifang Lin

Based on the Mie scattering theory and Maxwell stress tensor method, we investigate the transverse optical force (TOF) acting on chiral particles illuminated by a zero-order Bessel beam. It is demonstrated that the particle chirality can induce an azimuthal optical force (AOF), resulting in orbital motion of particles around the optical beam axis. The AOF depends strongly on particle loss as well as the handedness of chirality, with its amplitude capable of changing by over an order of magnitude by particle's chiral loss. The other component of TOF, the radial optical force (ROF), is much less sensitive to the magnitude and handedness of the particle chirality as well as the loss when the chirality is small. Analytical result based on dipole approximation reveals that the AOF arises from the direct coupling of particle chirality to both the spin angular momentum (SAM) and optical vorticity (curl of Poynting vector), exhibiting a conversion of optical SAM of an incident beam to mechanical orbital angular momentum of an illuminated particle. Differently, the ROF originates from the transverse gradient force. In addition, particle chirality yields a negative contribution to the gradient force; thus the ROF can be attenuated and even reversed in direction when particle chirality is sufficiently large. These characteristics of TOF might find applications in chirality detection as well as sorting chiral particles of different handedness and separating them from conventional ones.